(1) Field of the Invention
This invention relates to optical elements having antireflecting characteristics, especially to such elements in which the antireflection characteristic is provided by a modification of the reflecting surface.
(2) Description of the Prior Art
The desire to improve the performance of optical devices such as lenses and prisms by increasing the transmittance of light therethrough has been long appreciated. In particular, the prior art is replete with optical devices having surfaces coated with antireflecting layers, typically having an optical thickness of one quarter of a wavelength.
Less well known are optical devices in which surface reflections are reduced by altering the surface to provide a gradient in the index of refraction between that of the medium traversed by the incident light, such as air and that of the body of the optical device. One method for providing such an altered surface is disclosed in the Great Britain Pat. No. 29,561, and involves tarnishing glass surfaces in aqueous solutions of sulphuretted hydrogen in order to reduce the reflection of light therefrom. Subsequently, Nicoll (U.S. Pat. No. 2,445,238) disclosed a method for reducing reflections from glass surfaces in which the glass was heated in a vapor of hydrofluoric acid to form a skeletonized surface. Such treated surfaces are quite fragile and are easily damaged by contact with another surface. Furthermore, difficulties in reproducing the skeletonized structures and in maintaining a uniform structure over the entire surface area of optical devices has apparently led workers to develop alternative structures. Contemporaneously with the work of Nicoll, Moulton (U.S. Pat. No. 2,432,484) developed a technique for forming on glass surfaces a nonuniformly dispersed layer of collodial particles containing a random arrangement of peaks to provide the antireflecting characteristics.
It has recently been recognized that moths' eyes contain a regular array of conical protuberances which are believed to suppress reflections by providing a graded refractive index between the air and the cornea and thereby contribute to high degree of nocturnal sensitivity. Bernhard, C. G., Endeavor 26, pp. 79-84 (1967). This recognition has led to the suggestion that a glass lens having such a surface would exhibit similar reductions in reflectivity, but that considerable technological development would be required to make a process for forming such a lens practical. Clapham and Hutley, Nature, Vol. 244, p. 281 (Aug. 3, 1973). Thus, while recent investigators such as Clapham and Hutley appear to have rediscovered what Nicoll and Moulton had earlier noted, namely that a microscopically roughened glass surface or a microscopically rough layer on such a surface could exhibit reduced reflectivity, such recent investigators appear to have but produced a coated glass article having a regular surface pattern which is similar to a structure already known in nature, namely that of a moth's eye. See also U.S. Pat. No. 4013,465 (Clapham and Hutley).
With respect to optical devices formed of polymeric materials, even though such devices are of considerable technological importance, they have not been successfully produced with similar microstructured surfaces. If, in fact, previously known regularly structured surfaces may be used to emboss a structured pattern into softer materials, a great deal of work remained to be done before structures such as simulated moths' eyes can be mass produced. In U.S. Pat. No. 3,490,982 (Sauveniere and Doquire), it is suggested that a glass article having a treated surface may be used as a die for making a plaster cast from which a positive replica could be produced on a thermoplastic material. While such processes are speculated, there has been no successful production or exploitation of optical devices formed of such materials.